It is known that metal halogenides, which are in fact ozone-sensitive, exhibit a strong hygroscopic behavior. This leads to a major problem when metal halogenides are used as gas-sensitive layers.
The strong hygroscopic behavior of ozone-sensitive halogenide salts such as potassium iodide (KI) or sodium iodide (NaI), for example, means that a gas-sensitive layer produced from these salts is unstable and results in the dispersion of this layer upon reaction with water. Consequently, the sensitive layer loses its structure, which leads to a loss of sensitivity culminating in total insensitivity. Previously, this meant that sodium iodide could not be used as a gas-sensitive layer. Furthermore, the gas absorption of the layers with ozone leads to a transformation of the sensitive layer material and hence to a change in the surface morphology. Studies revealed that the ozone treatment led after a certain period of time to the occurrence of layer instabilities, including shrinking of the material, agglomeration of the salt crystals, and separation of the layers. For this reason the layer becomes increasingly damaged already after, for example, six weeks in service, which is associated with a continuous reduction in sensitivity. As a result of the strong hygroscopic behavior, the ozone measurements based on metal halogenides exhibit a high cross-sensitivity at least to humidity. This means that the measurement or sensor signal changes with different humidity concentrations analogously to the ozone concentration and therefore leads to a falsely positive ozone signal or else too high an ozone signal is simulated. For this reason a correction of the ozone signal relative to the response to humidity or else to other interfering influences is essential.
Gas-sensitive layers are not suitable for use in gas sensor technology if they do not exhibit sufficient stability. The previously described ozone-sensitive materials usually exhibit a life of about 12 weeks. However, for practical application in the end consumer sector it is essential that the sensor exhibits a longer service life and also a lower cross-sensitivity to humidity.
Previous layer preparations for ozone-sensitive layers were based in particular on vapor deposition on a substrate of a sensor body, a technique which is described in particular in [1]. Layers of this type exhibit the above-described problems in particular.
To solve these problems, measures such as those described in particular in [2] have already been taken previously.
For example, an attempt was made to overcome the problems by variation of the ozone-sensitive salts. When KI is used in a humid atmosphere, the strong hygroscopic behavior of the material and the resulting formation of potassium hydroxide (KOH) lead to dispersal of the layer. No improvements could be achieved in this regard by the use of other possible ozone-sensitive salts, for example other alkali halogenides (NaI, LiI) or alkaline earth halogenides (CaI2, SrI2). Firstly, some of these halogenides are no longer stable in air; secondly, some of these compounds exhibit a much stronger hygroscopic behavior than KI itself. As a result, layer preparation is precluded from the outset.
Another measure to improve the problems cited consists in pretreatment of the substrate surfaces with lithium hydroxide (LiOH). Pretreatment of the SiO2 surface with concentrated LiOH solutions causes free bonding points to be activated at the substrate surface. Following intensive ozone treatment it was shown that although a somewhat stronger bonding of the KI to the substrate surface could be achieved than, for example, on a substrate coated with platinum, the surface coating was porous.
Artificial roughening of the surfaces, for example, was used to enlarge the total surface area, with the aim of achieving improved bonding and coating of the surface. When this is done, however, the layer is similarly damaged following ozone treatment and some of the KI crystals migrate to the outside.
A further improvement was anticipated through the use of porous silicon, onto which KI was applied by vapor deposition. In a subsequent treatment with ozone, it was revealed that a layer prepared by this method exhibited the most stable surface adhesion. However, since the silicon has to be metallized, the good adhesion properties are reduced due to the flattening of the pores.
Another variation on eliminating the cross-sensitivity to humidity consists in the use of a separate humidity measurement by means of an external humidity sensor in order to correct the actual gas sensor signal.